Underdog Japanese player Kei Nishoriki stunned the US Open today as he defeated Novak Djokovic to become the first Asian player ever to reach a grand slam final.

He was watched by a host of stars including pregnant Chelsea Clinton, who tried to beat the heat in a white shirtdress as she was joined by her husband Marc Mezvinsky.

In a remarkable display of endurance, Nishikori followed up punishing five-set wins over third seed Stan Wawrinka and fifth seed Milos Raonic with an even more extraordinary effort, grinding down the top-ranked Djokovic in two hours, 52 minutes on a sweltering Arthur Ashe Stadium court.

'It's just amazing, an amazing feeling beating the number one player and also playing much better in these couple of matches so I'm so happy,' the 10th-seeded Nishikori said.

Underdog: Tenth seed Kei Nishikori, from Japan, jumps for joy after he beat number four seed Novak Djokovic in five sets amid soaring heat at the US Open

Groundbreaking: The Japanese star, 24, became the first Asian man to reach a men's singles final in a major tournament

Flagging: Djokovic said he was 'not himself' as he was outplayed in the soaring heat

Enjoying the show: Chelsea Clinton and her husband Marc Mezvinsky watched Novak Djokovic of Serbia play Kei Nishikori of Japan at the US Open today

Getting close: The former First Daughter and her husband looked very much in love as they soaked up the sun

'It was really tough conditions today, felt a little bit heavy and humid too but I guess I love to play long matches and I hope I can recover well for the final.'

The midday sun beat down on Arthur Ashe Stadium and a thermometer on court showed the temperature nearing 100 degrees (37 Celsius), not counting the humidity of close to 70 percent.

The 24-year-old Japanese awaits the winner of the second semi-final between world number two and five-times U.S. Open champion Roger Federer and 14th seed Marin Cilic in Monday's final.

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"I don't know what's going on," said Nishikori, who is coached by former grand slam champion Michael Chang. "I was a little bit tight; especially it was my first semi-final in a grand slam."

Djokovic was magnanimous in defeat.

"It was not easy to play in these conditions but also he had more hours spent on the court. So it's no excuse," the Serb said.

"I expected him to be able to play another five-setter because he had two days off.

"He just played better in these conditions than I did.

"I just wasn't managing to go through the ball in the court. You know, I wasn't in the balance. Unforced errors. Even when the ball gets back to his part of the court it's pretty short; he takes advantage of it. On the other side I didn't. That's it."

Tender moment: Mezvinsky also kissed his wife's hand as they watched the tennis in humid temperatures in the high 80s

Vogue editor Anna Wintour was also in Flushing to watch Roger Federer take on Marin Cilic in the semi-finals. The Swiss player's wife Mirka, right, was also there to cheer on her husband

Nishikori arrived at sun-bathed Arthur Ashe Stadium looking fresh despite having played the latest finishing match ever at the U.S. Open on Tuesday in the fourth round against Raonic, when he walked off court at 2:26 a.m. local time.

Two days later Nishikori was forced to go the distance again, out-lasting Wawrinka in a four hour, 15 minute battle.

But the biggest test of all came with gusting wind, stifling humidity and courtside temperatures hovering around 100 degrees (Fahrenheit), turning the semi-final into a survival of the fittest against a surging world number one who had dropped just one set on route to the final four.

A day earlier, Jenna Bush Hager matched her outfit with the occasion when she appeared yesterday at New York's US Open to watch the quarter-finals, dressed in a monochrome pleated minidress.

The 32-year-old Today show correspondent and daughter of former president George W Bush was all smiles, and cheered animatedly as Swiss champion Roger Federer beat French rival Gael Monfils to secure a place in the semi-finals.

Mrs Bush Hager, who left her 16-month-old daughter Mila at home, was on a double date with husband Henry Hager, as well as friends Colleen Dixon, a lawyer, and Graves Tompkins, a banker.

She posted a grinning snap to her Instagram, in which she poses with her husband and friends, captioned: 'Bad selfie watching good tennis at US Open! Fun!'

A long-time tennis fan, Mrs Bush Hager was seemingly enthralled in the action; pictured at the edge of her seat as the tense game drew to its conclusion.

From the looks of things, the mother-of-one was well-rested, just a day after she blamed 'sleep-deprivation' on stumbling into something of a political faux-pas.

The next month there was more photo jubilation, this time for the former president's 68th birthday, in which the hands-on grandfather is seen tenderly feeding Mila a bottle in an Instagram snap, captioned: 'Happiest birthday to my old man and Mila's Jefe.'

Enthralled: Jenna Bush (left) attended the US Open quarter-finals yesterday in New York with her husband and friends, and cheered enthusiastically as Swiss Roger Federer beat French rival Gael Monfils

Watch that breeze! The 32-year-old Today show correspondent paired her outfit with the occasion, teaming an athletic pleated minidress with a Chanel bag (pictured)